This invention relates to an electrochromic display device, which utilizes a combination of two kinds of electrochromic materials and can be operated so as to selectively and alternately exhibit at least two different colors aside from colorless transparency.
Electrochromic (EC) display devices utilize reversible coloring and bleaching that accompany reversible electrochemical oxidation and reduction of selected materials. Compared with liquid crystal display devices, EC display devices have advantages such as no need of polarizing plate, viewability from any angle and clearer coloration.
Conventional EC materials are classified into two types, which will be referred to as "oxidation coloring type" and "reduction coloring type", respectively, in the present specification. An EC material of the oxidation coloring type takes on color in its electrochemically oxidized state, whereas an EC material of the reduction coloring type takes on color in its electrochemically reduced state.
It is known to use a combination of an oxidation coloring type EC material and a reduction coloring type EC material in an EC display device having oppositely arranged two electrode layers. The two types of EC materials are assigned to the two electrode layers, respectively. For example, JP-A No. 59-159134 shows using a combination of Prussian blue which assumes blue color in its electrochemically oxidized state and becomes colorless by reduction and tungsten trioxide which is colorless in its electrochemically oxidized state and assumes blue color in a reduced state. In operation of the display device, electrochemical oxidation of the EC material of one electrode is accompanied by electrochemical reduction of the EC material of the opposite electrode. Accordingly simultaneous coloration of the two electrodes and simultaneous bleaching of the two electrodes take place. The primary purpose of this construction is intensifying blue coloration of the display device.
There is an increasing demand for EC display devices that can exhibit at least two different colors, but the demand has not been met from an industrial point of view. There is a proposal of using a combination of two kinds of organic dyes one of which belongs to the oxidation coloring type and the other to the reduction coloring type. However, EC devices using such dyes are deemed impracticable because of insufficient stability and poor endurance to repeated driving.